1. Field of the Invention
The present invention pertains to the field of satellite communications. More particularly, the present invention relates to satellite-communication networks comprising a master earth-station and a number of remote earth-stations. In particular, this invention pertains to the control of the signal power level transmitted by the earth-stations in such networks.
2. Description of the Related Art
FIG. 1 shows a conventional very small aperture terminal (VSAT) satellite-communication network. The VSAT network comprises a master earth-station, referred to herein as a "hub" 10, a number of remote earth-stations, referred to herein as "VSATs" 20, and a geostationary communication satellite transponder, referred to herein as a "satellite" 30. The hub 10 communicates with the VSATs and the VSATs 20 communicate with the hub 10 by sending transmission signals 40 through the satellite 30.
FIGS. 2A through 2D illustrates the operation of the conventional VSAT satellite communication network of FIG. 1. As these figures show, the communication between the hub 10 and the VSATs 20 is accomplished through the use of an outbound transmission signal from the hub 10 to the VSATs 20, and an inbound transmission signal from the VSATs 20 to the hub 10.
FIGS. 2A and 2B illustrate an outbound transmission signal from the hub 10 to a VSAT 20. As shown in FIG. 2A, the outbound transmission signal first includes an outbound uplink portion 210, passing from the hub 10 to the satellite 30. As shown in FIG. 2B, the outbound transmission signal also includes an outbound downlink portion 220, passing from the satellite 30 to the hub 10 and all VSATs 20.
FIGS. 2C and 2D illustrate an inbound transmission signal from a VSAT 20 to the hub 10. As shown in FIG. 2C, the inbound transmission signal first includes an inbound uplink portion 230, passing from a VSAT 20 to the satellite 30. As shown in FIG. 2D, the inbound transmission signal also includes an inbound downlink portion 240, passing from the satellite 30 to the hub 10.
The outbound transmission signal 210 and 220 is a continuous signal sent from the hub 10. In contrast, the inbound transmission signal 230 and 240 is sent in bursts as needed by the various VSATs 20.
Satellite transponder resources are sold and leased in units of power and bandwidth. A VSAT network operator must carefully control both resources in order to achieve economical operation.
Code-division multiple-access (CDMA) is a multiple-access technique that forms the basis for the IS-95 digital cellular telephony standard, and has some important advantages for use in VSAT networks, particularly when it is important to be able to use small antennas at the remote terminals. The first widely-deployed VSAT networks using CDMA used the C200 product developed by Equatorial Communications Company (ECC) of Mountain View, Calif.
It is widely recognized that accurate power control is required to equalize the received power levels of signals multiplexed on a channel using the CDMA technique to maximize the operational efficiency of the network. Qualcomm, Inc. has developed a number of power control techniques for use in the CDMA cellular telephony networks that they have developed based on the IS-95 standard. Qualcomm's techniques are designed for terrestrial networks that operate without a satellite relay, and are designed to cope with the rapid fading that occurs in a mobile terrestrial microwave propagation environment.
Operational experience with the ECC C200 networks also showed the need for accurate power control of the VSAT transmitters. The C200 product was limited by the fact that the control of the VSATs' inbound (VSAT-to-hub) transmitted power levels had to be accomplished by manual intervention of an operator at the hub. Since the C200 system was primarily designed for C-Band operation, the rapid fading that occurs at higher frequencies due to rain was not a serious problem for the system. However, VSAT networks based on this product typically required periodic expert rebalancing of the inbound power levels across the network to compensate for gradual changes in the equipment or haphazard adjustments by inexperienced operators.
For VSAT networks operating at Ku-Band and higher frequencies, rain fade is a serious problem. Rain fade results from the absorption and scattering of the transmission signals 40 between the hub 10 and satellite 30 and between the VSATs 20 and the satellite 30 by water droplets or ice crystals in the atmosphere. During rain fade, changes in attenuation and hence the received signal level can occur within a few seconds. At Ku-Band and higher frequencies, rapid and automatic uplink power control becomes very important.
Uplink power control has typically been implemented only on the outbound (hub-to-VSAT) link, where the additional cost of the equipment at the hub is of minor consequence. A rain fade affecting the outbound uplink (a rain fade between the hub 10 and the satellite 30) affects the entire network, while a rain fade on the inbound uplink (a rain fade between a VSAT 20 and the satellite 30) only affects that VSAT 20. Standard practice has been to operate the VSATs 20 with enough inbound (VSAT-to-hub) power to overcome most rain fades.
For CDMA VSAT operation at Ku-Band and higher frequencies, however, uplink power control on the inbound signal becomes a necessity. Uplink power control can also benefit TDMA and other modes of satellite access operation by providing the network operator the ability to control and thus reduce his transponder power requirement, by only operating the VSAT transmitter at high power levels when required to overcome rain fades.